The subject of the invention is a method and apparatus for rolling laterally ribbed bimetallic pipes, especially with small thicknesses of ribs, finding the application in heat exchangers.
Rolling the ribs on tubes consists in pressing down rotationally driven multi-disk tools to the surface of the tube. Usually a set of three tools is used, comprising sets of disks realizing several stages of producing the rib, which are progressively deforming the tube, drawing the rib and forming the shape of the rib.
In the case of producing bimetallic tubes additionally the external ribbed tube is pressed down to the internal core tube in order to provide for good adhesion of tubes and an improvement of thermal conduction.
Those skilled in the art know a method of rolling the ribs, in which the external tube is pressed down to the internal tube in the last stage of forming the shape of ribs. In this method described for example in the USSR patent specification no. 217344 all tools are provided with the last - in the direction of rolling - disks of an increased diameter, causing the formation of additional radial forces which causes pressing down the tubes. It has been observed, however, that in the result of the action of friction forces and the increase of the perimeter of the rib being thinned, radial forces of the opposite direction appear therein, which causes tearing off the material of the external tube from the internal tube and, in consequence, formation of a sub-rib draw-in, and thus, a gap between the tubes.
Another tool for rolling ribs in bimetallic tubes is known from the Polish patent specification No. 79817. In the said tool, in order to eliminate the inter-tube gap a set of unsymmetrical plano-convergent disks is additionally applied. Such a design of the tool enables in the first stage the formation of a low rib, and then intensive billateral thinning thereof with simultaneous increasing the height thereof. The thinning process creates the tendency to increase the perimeter of the rib, whereas limiting the radial flow of metal in the region adjacent to the tops of initial disks leads to the formation of the draw-in directly under the rib. The set of unsymmetrical disks performs the work causing the previously formed draw-in to be shifted to the region of the direct action of the last burnishing disk.
Realization of the above presented method of pressing down the external tube to the internal tube by means of several unsymmetrical disks causes considerable loads along the axis of the tool, which hinder the practical utilization of the said solution in rolling ribbed tubes of very small pitches and small thicknesses of ribs.
Those skilled in the art know also from the Polish patent specification No. 113419 a method of rolling ribs by introducing after initial deforming the tube, drawing and forming the rib, additional operations consisting in annealing and then flexing several-times-repeated the ribs in relation to the axis perpendicular to the direction of rolling, and pressing down the external tube to the core tube during contraflexure. The tool for realization of the said method comprises, apart from the known sections of initial disks, a supporting disk increasing the rigidity of the tool and initially deflecting the rib, a section of counter-flexure disks at a certain distance from this disk, inside which there are burnishing disks, and also, as the last one, the disk for straightening the rib. By the method and the tool according to the said invention it is possible to carry out industrial rolling of ribs having an average thickness not smaller than 0.45 mm and an average slenderness ratio of up to 1:40. At further thinning the rib a significant influence on the formation of the rib is gained by certain defects, included in production standards for pipes, on the surface of smooth charge tubes. These defects cause local cracks in the rib and as result lead to destruction of the rib band in the stage of multiple contraflexure.